The FDM numerical simulation software, View-Cast system, was employed to simulate the mould-filling and solidification process in low pressure die casting (LPDC) of Al-alloy motor enclosure. The distribution of liquid fraction, temperature field and the solidification pattern of casting were revealed and the formation mechanism as well as distribution rule of the potential defects, namely gas pores and shrinkage pores, were predicted. Furthermore the solutions to eliminate them were presented. By reducing the pouring rate, the turbulent flow at the corner of casting had eased, so the gas pores could be eliminated. Then the force-cooling system was installed on the top of mould, which could reduce the volume of liquid islands efficiently so that the shrinkage defects can be decreased to its limit.